1. Field of the Invention
The invention relates to zig-zag sewing machines and is particularly directed to needle bar drive mechanism therefor.
2. Description of the Prior Art
During stitch formation in a sewing machine the needle bar is reciprocated endwise to penetrate the work and from the point of maximum penetration, it rises to throw a loop on the underside of the work that is then entered by the loop seizing beak of a hook. There is an optimum delay, such as twenty degrees of arm shaft rotation, between the time the needle reaches the point of maximum penetration and the time that the beak enters the loop which is calculated to produce a needle thread loop that is sufficiently large to insure loop seizure and to position the eye of the needle in a specific relationship to the loop seizing beak for the proper presentation of the loop to the loop seizing beak.
In straight-stitch machines timing of the needle relative to the hook for proper loop taking doesn't present a problem. However, in zig-zag sewing machines the optimum timing relationship is effected by lateral movement of the needle. When the needle is moved in one direction, loop seizure is retarded by the amount of time required for the loop taker to travel from the previous position of loop seizure to the new position of loop seizure. Since the needle bar continues to rise during the time that the loop taker is traveling to the new position, the optimum timing relationship is destroyed, or more specifically, the eye of the needle is too high. Conversely, when the needle is moved in the other direction, loop seizure is advanced with the result that the eye of the needle is too low.
One approach to the timing problem in zig-zag machines is to time the needle relative to the hook so as to obtain the optimum relationship in the zero bight position, or in other words in a position in the center of the zig-zag pattern. The effects of the variations in the time of loop seizure are thereby minimized. Nevertheless, variations from the optimum timing relationship during zig-zag operations result in skipped stitches due to failure of the hook to seize the loop, and also result in thread breakage because of fouling of the loop.
Another approach to the timing problem is to provide a needle drive which compensates during zig-zag operations for variations in the timing of the needle relative to the hook in such a way as to cause the needle to be disposed at the optimum height relative to the hook for loop seizure in zig-zag positions of the needle. Compensating linkage arrangements have been proposed for such purpose but have proved to be unsatisfactory because of accompanying force components which interfered with proper operation of the gate serving to move the needle into zig-zag positions, and caused undue wear on parts of the needle drive mechanism. An example of a prior art compensating linkage arrangement is disclosed in U.S. Pat. No. 2,932,268 of The Singer Company for "Needle Bar Drives For Zig-Zag Sewing Machines" issued Apr. 12, 1960.
It is a prime object of this invention to provide the needle drive of a zig-zag sewing machine with a linkage arrangement which is effective to cause a needle to be properly positioned for loop seizing in zig-zag positions of the needle and which operates without causing undesirable forces to be applied to the gate of the machine or to parts of the needle drive mechanism.